1. Field of the Invention
The present invention relates to an electrochemical reactor cell stack and to an electrochemical reactor system such as a solid oxide fuel cell formed of such a reactor cell stack, and more specifically relates to an electrochemical reactor system employing tube electrochemical reactor cells which are capable of dramatically increasing the output per unit volume through the use of specific electrochemical reactor bundles and stacks. The present invention provides novel techniques and novel products related to electrochemical reactor cell bundles and stacks as well as electrochemical reactor systems employing such reactor cell stacks, which are suitable for use as clean energy sources and environmental purification devices.
2. Description of the Related Art
Solid oxide fuel cells (SOFC) are known as typical electrochemical reactors. An SOFC is a fuel cell in which an ion-conductive solid oxide electrolyte is used as the electrolyte. The basic structure of the SOFC is usually composed of three layers including a cathode (air electrode), solid acid electrolyte, and anode (fuel electrode), and is normally used at an operating temperature range of 800 to 1000° C.
When a fuel gas (such as hydrogen, carbon monoxide, or a hydrocarbon) is supplied to the anode of the SOFC and air, oxygen or the like is supplied to the cathode, a difference occurs between the oxygen partial pressure of the cathode and the oxygen partial pressure of the anode, and voltage is thereby produced across the electrodes according to the Nernst equation. The oxygen is converted to ions at the cathode, which travel through the interior of the dense electrolyte to the anode, and the oxygen ions which reach the anode react with the fuel gas, releasing electrons. As a result, a load is applied to the anode and cathode, allowing electricity to be directly extracted.
To make a more practical SOFC, the SOFC operating temperature will have to be lowered, and to achieve that, it may be effective to form the electrolyte into a thin film and use an electrolyte material with high ion conductivity. Anode support-type cells have been widely researched because the use of a support made with an electrode material allows electrolytes to be formed into thin film. Lowering the operating temperature to between 500 and 600° C. is expected to allow less expensive materials to be used and operating costs to be reduced, thereby expanding SOFC applications.
Flat types of SOFCs with a high power output of 0.8 to 1 W/cm2 at a low temperature (600° C.) have thus far been reported with the proposal of new anode and cathode materials (Z. Shao and S. M. Haile, Nature 431, 170-173 (2004); T. Hibino, A. Hashimoto, K. Asano, M. Yano, M. Suzuki and M. Sano, Eletrochem. Solid-State Lett., 5(11), A242-A244 (2002)).
However, the anode support-type SOFCs with high power output that have been reported thus far are the flat type, which are susceptible to cell failure under extreme operating cycle conditions. That is because the cells become deformed and fail as a result of the considerable change in volume in commonly used nickel cermet due to temperature changes and cycling in oxygen-reducing atmospheres.
An extremely important technical issue is therefore to find a way of enlarging and stacking flat SOFCs while preserving flat cell performance. Controlling the electrode structure of the anode support substrate and making it thinner are important in terms of improving performance, but it has been difficult to further reduce the thickness and increase the porosity of flat types. SOFC structures consisting of tubular cells have been researched as alternatives to flat cells (JP 2004-335277 A).
The tube cell bundles and stacks that have been proposed thus far have a structure in which the tube cells are stabilized and held by stacking structures composed of a cathode material, and are the kind in which current is collected from the anode and cathode using electrode-collector sheets and the like.
However, in conventional tubular cell bundles and stacks, the ends of the tubes are secured by collector parts (see FIG. 1), and the current design makes it difficult to join the tube ends with the collector parts, resulting in problems such as greater contact resistance, and less thermal shock resistance because the tubes are secured by collector parts. In addition, ever smaller tube diameters have led to demand for higher tube positional precision. This has led to greater difficult in controlling the position when electrode-collector plates are attached, and so far no new techniques allowing such problems to be overcome have yet been proposed in this technical field.
Under these circumstances, the inventors undertook extensive research in view of the prior art noted above to develop a novel type of SOFC and novel uses capable of definitively solving the problems of conventional parts involved in securing the above tube tips with collector parts. As a result, the inventors developed a novel collector method for bundle structures comprising the arrangement of micro-diameter tube cells as well as a stacking method using them, and perfected the present invention through further research upon finding that these stacks can be used to construct a novel electrochemical reactor system allowing the operating temperature to be lowered, etc.